Since the advent of the rolling cutter rock bit for earth boring, a large number of different tooth shapes have evolved for improving rate of penetration and life expectancy in the variety of specific geological formations for which they were intended. Exemplifying insert type cutter teeth of the prior art are U.S. Pat. Nos. 3,442,342 and 4,086,973. Disclosed in those patents are specially shaped tungsten carbide inserts for rotary cutter rock bits with the '342 patent disclosing a specific shape suitable for drilling hard abrasive formations. The '973 patent discloses a shape more readily adapted for drilling formations of medium hardness, e.g., hard shales, dolomite and some limestones. Further disclosed in the '342 patent are two cutter inserts having tips representing a modified chisel with convex flanks converging to a crest while the '973 patent discloses a wedge shape insert in which the flanks are twisted or canted away from each other. Rounded intersections are provided to avoid sharp corners and sharp edges which can cause high stress concentration to be incurred.
More suitable for relatively soft or medium earth formations is the insert shape of U.S. Pat. No. 4,108,260. As therein disclosed, the insert is generally chisel shaped with asymmetrical flanks converging to a crest. The leading flank is scoop-shaped and the trailing flank is rounded outwardly.
In U.S. Pat. No. 2,774,570 there is disclosed an annular series of cylindrical inserts of hard wear-resistant material. The insert axes extend outwardly and substantially normal to the surface of the body and present protrusions at the surface to effect a disintegrating action while maintaining gage of the well bore being drilled. Numerous other shapes and configurations for various intended purposes are likewise known. See for example U.S. Pat. Nos. 2,121,202; 3,339,431; 3,495,668; 3,743,038; 3,388,757; 4,058,177; 4,108,260; and 4,168,923. It is of course important to bear in mind that the ultimate objective with respect to such cutters is to achieve a high rate of penetration into the particular earth formation for which the cutter design is intended. At the same time it is important to realize long term life expectancy from such bits whereby a maximum penetration depth is achieved in the shortest possible time before replacement of the cutter becomes necessary.
The cutting structures disclosed in the foregoing patents have various degrees of merit in achieving their sought after objectives, but none are specifically adapted for maximizing the rate of penetration and durability in relatively soft geological formations such as soft clays, sandy clay, sandstones or marl (those having low compressive strength or those tending toward plastic behavior). Moreover, while the relationship between extension length of the insert and the cutter penetration rate in soft formations is well known, the use of greater extension lengths have not previously been utilized. This is generally attributed to the likelihood of metal-to-metal interference between the insert and the adjacent cutter with constructions of the prior art; the greater potential for insert wear of breakage and the desire to maintain present center-to-center dimensional placement arrangements for maximization of inserts.
Also vital in soft formation cutting is the need for adequate crest length for earth removal with a scraping action once the cutter has effected penetration. By virtue of the geometric configurations of conventional insert cutter tips, efforts to increase the extension to a maximum have resulted in a crest length that is proportionally reduced with a correlated reduction in insert strength. Consequently, mere extension of prior art constructions to achieve longer overall length with an increase in crest length has required an increased insert diameter in order to maintain even a constant crest length. With a larger diameter the crest length occurs further removed from the base, thus contributing to an increased extension. At the same time the larger diameter has the undesirable effect of reducing the number of inserts placeable about the cutter (cone) surface. If reduced to an insufficient number, the cutters become operationally ineffective or inefficient for removing the formation. Alternatively, if the extension of the insert is increased, it potentially produces the above mentioned interference between adjacent cutters, which in turn, requires either relocating the insert or modifying the material thickness surrounding the insert. The latter is of course vital to the strength and integrity of the cone against fracture and erosion wear and both are usually avoided to ensure adequate insert retention in its socket. Likewise the insert location is important for ensuring proper stress loading of the insert and the rock bit bearings while rendering the bit operationally effective for proper removal of the formation across the hole.
Despite recognition of the foregoing problems, a ready solution therefor has not heretofore been known.